@charset "UTF-8";
/* register these two so they can be transitioned */
/* index of previous top item during the animation
 * after another has been selected */
@property --p {
  syntax: "<number>";
  initial-value: 0;
  inherits: true;
}
/* animated index of top item from previous to current */
@property --v {
  syntax: "<number>";
  initial-value: 0;
  inherits: true;
}
* {
  margin: 0;
}

/* poor man's reset */
/* grid everything! */
html, body, section, article, button {
  display: grid;
}

/* full viewport height container for stack section */
html {
  min-height: 100%;
}

body {
  background: #070410;
}

section {
  /* set previous top item index to current top index
   * 0s duration delayed transition ensures switch happens 
   * after the switching top item transtion */
  --p: var(--k);
  /* abs of difference between current & previous top item indices
   * 0 if not during animation, 
   * 1 during animation not between two ends, 
   * n - 1 > 1 (if n > 2) during animation from one end to another */
  --abs-p: abs(var(--k) - var(--p));
  /* animation from one end of the n > 2 length list to another?
   * 0 if no, 1 if yes */
  --end: clamp(0, var(--abs-p) - 1, 1);
  /* direction we're going in given by 
   * sign of difference between current & previous top item indices
   * -1 backwards, 1 forwards, 0 no animation;
   * switch sign IF we're going from an end to another */
  --dir: calc((1 - 2*var(--end))*sign(var(--k) - var(--p)));
  /*  forward direction flag: 0 backwards, 1 forwards */
  --fwd: calc(.5*(1 + var(--dir)));
  /* set animated value of top item index to current top index
   * a transition ensures switch doesn't happen instantly */
  --v: var(--k);
  /* absolute value of difference between animated & previous top index */
  --abs-v: abs(var(--v) - var(--p));
  /* animation progress as a decimal value */
  --prg: calc(var(--abs-v)/(1 - var(--end) + var(--end)*(var(--n) - 1)));
  /* bigger space between the two columns (image stack & all else) */
  grid-gap: 0.5em 4em;
  grid-template: repeat(2, max-content) 1fr max-content/max-content 1fr;
  place-self: center;
  color: #f1f5f9;
  font: 1em poppins, sans-serif;
  counter-reset: k calc(1 + var(--k)) n var(--n);
  /* transition previous & animated top item indices 
   * so they don't change instantly like the current one */
  transition: --p 0s 0.8s, --v 0.8s;
  /* counter & binomial name faded */
  /* this ridiculousness needed for Chrome without flag */
  /* for reference
   * https://css-tricks.com/using-absolute-value-sign-rounding-and-modulo-in-css-today/ */
}
section::before, section em {
  color: RGB(from currentColor r g b/0.6);
}
section::before {
  /* counter */
  grid-area: 1/2;
  /* all grid areas need to be set manually */
  content: counter(k) "/" counter(n);
}
@supports not (scale: Abs(-2)) {
  section {
    --abs-p: max(var(--k) - var(--p), var(--p) - var(--k));
    --abs-v: max(var(--v) - var(--p), var(--p) - var(--v)) ;
  }
}
@supports not (scale: Sign(-2)) {
  section {
    --dir: clamp(-1, (var(--k) - var(--p))*100000, 1) ;
  }
}

article {
  /* absolute value difference between 
   * currently top item index and current item index */
  --abs-top: abs(var(--k) - var(--i));
  /* not top item if the absoute value difference ≥ 1
   * top if the difference is 0 */
  --not-top: min(1, var(--abs-top));
  /* top flag is the negation */
  --top: calc(1 - var(--not-top));
  /* difference between moving image index which is 
   * previous top item index --p if going backwards, 
   * current top item index --k if going forwards 
   * and current item index --i */
  --val-mov: ((1 - var(--fwd))*var(--p) + var(--fwd)*var(--k) - var(--i));
  --abs-mov: abs(var(--val-mov));
  /* not moving image if the absoute value difference > 1
   * moving image if the difference is 0 */
  --not-mov: min(1, var(--abs-mov));
  /* moving flag is the negation */
  --mov: calc(1 - var(--not-mov));
  grid-area: 1/1/-1/-1;
  /* stack to occupy entire parent grid */
  grid-template: subgrid/subgrid;
  /* and inherit template */
  /* debatable whether z-index is the best way 
   * maybe CSS 3D transforms would be a better idea */
  /* depends on number of items, its own index and top item index */
  z-index: mod(calc(var(--n) - 1 + var(--i) - var(--k)), var(--n));
  /* transition z-index */
  transition: z-index 0.8s cubic-bezier(1, -0.9, 0, 1.9);
  /* this ridiculousness needed for Chrome without flag */
}
@supports not (scale: Abs(-2)) {
  article {
    --abs-top: max(var(--k) - var(--i), var(--i) - var(--k));
    --abs-mov: max(var(--val-mov), -1*var(--val-mov));
  }
}

h2, em {
  /* fade out and slide down when not top item anymore,
   * delayed fade in and slide up when becoming top item */
  translate: 0 calc(var(--not-top)*1lh);
  opacity: var(--top);
  transition: 0.4s calc(var(--top)*.5*0.8s);
  transition-property: translate, opacity;
}

/* need to specify grid-area for all */
h2 {
  grid-area: 2/2;
}

em {
  grid-area: 3/2;
}

img {
  /* value that grows from 0 to 1, then goes back to 0 
   * during the switching top items transition */
  --sin: sin(var(--prg)*.5turn);
  grid-area: 1/1/-1;
  /* occupy entire first column */
  /* slight glow effect */
  border: solid 2px rgba(82, 82, 122, 0.5);
  /* whatever, maybe better ways for setting dims */
  height: 13em;
  aspect-ratio: 1;
  object-fit: cover;
  /* fill with no distortion */
  border-radius: 0.75em;
  /* slide just moving image out & back in */
  translate: calc(-150%*var(--mov)*sqrt(var(--sin)));
  /* random rotation, but not during slide out & back in transition */
  rotate: calc((1 - var(--sin))*var(--a));
}

div {
  /* button wrapper */
  display: flex;
  gap: 2em;
  grid-area: 4/2;
  /* prevent button clicks during animation */
  z-index: calc((1 - min(1, var(--abs-p)))*var(--n));
}

button {
  /* prettify button */
  --sgn: -1;
  --prc: calc(var(--hov, 0)*100%);
  --c: color-mix(in hsl, #818cf8 var(--prc), #52527a);
  border: none;
  width: 1lh;
  aspect-ratio: 1;
  border-radius: 50%;
  background: RGB(from var(--c) r g b/0.2);
  color: color-mix(in hsl, #818cf8 var(--prc), currentcolor);
  font: 900 2em/1.5 sans-serif;
  transition: 0.3s ease-out;
  transition-property: background-color, color;
}
button::before {
  /* arrow, SVG likely better, whatever */
  place-self: center;
  border: solid 2px;
  border-width: 2px 2px 0 0;
  width: 35%;
  aspect-ratio: 1;
  translate: calc(var(--sgn)*-15%);
  rotate: 45deg;
  scale: var(--sgn);
  content: "";
}
button[data-inc="1"] {
  --sgn: 1;
}
button:is(:hover, :focus) {
  --hov: 1 ;
}